Radical grafting of functional monomers onto polyolefins is done either in the molten state or in solution, using radical initiators such as peroxides, or in the solid state, by irradiation. Owing to the action of the radicals, secondary reactions take place at the same time as the grafting reaction. They result in an increase in the molecular mass in the case where the polymer to be grafted is polyethylene, or to its decrease in the case where it is polypropylene. If the quantity of radicals needed for the grafting reaction is large, the change in the molecular mass of the polyolefin produces a considerable change in its melt viscosity. These phenomena reduce the quantity of reactive functional groups that can be incorporated in the polyolefin by radical grafting of functional monomers.
Applicant has discovered the advantage of employing stable free radicals when polyolefins are being grafted with a functional monomer. It makes it possible to limit scission reactions in the case of polypropylene and crosslinking reactions in the case of polyethylene. In both cases this results in an improvement in processing, for example by extrusion, of the grafted products.
At the same graft content, PP (polypropylene) grafted in the presence of stable radicals is more viscous; it therefore behaves better as a melt and is easier to granulate.
Polyethylene grafted in the presence of stable radicals is less viscous; when it is extruded, the pressure at the extruder head is reduced and its processing is easier, even at high graft contents.
The prior art U.S. Pat. No. 5,344,888 has described the grafting of polypropylene homo- and copolymers with maleic anhydride in the presence of peroxide initiators and in the presence of a coagent which may be, for example, diallyl maleate, diallyl succinate or triallyl cyanurate. These are not stable free radicals, nor generators of such radicals. The examples show that it is possible to obtain polypropylene containing up to 2% by weight or more of grafted maleic anhydride.
However, although this process allows large quantities of maleic anhydride to be grafted, it does not enable the deterioration of viscosity to be avoided.
The prior art EP 726 289 describes generalities relating to the stabilization of polymers with stable free radicals, for example during their grafting. The examples show the grafting of styrene butadiene diblock copolymers with mercaptopropionic acid or methacrylic acid.
Grafting in the absence of stable free radicals results in a polymer containing 2.8% of grafts, but which is crosslinked when compared with the initial polymer, whereas grafting in the presence of stable free radials avoids the crosslinking of the initial polymer, but the graft content drops to 2.4%.
This prior art shows either the protection of the styrene polymers or of the PMMA by stable radicals (TEMPO) to avoid their depolymerization during their melting, or the grafting of acids onto styrene block copolymers. The quantities of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) are 10 millimoles per kg of polymer which is being grafted (i.e. 0.15%). The description mentions a range of from 0.05 to 0.5% of stable radicals in relation to the weight of the polymer to be grafted and of the graft unit. These quantities correspond to a range of from 3.5 to 33 millimoles of TEMPO per kg of the total polymer to be grafted and of the graft unit.
Grafting of polyethylenes and of polypropylenes with functional monomers and, in particular, unsaturated carboxylic acid anhydrides, is not exemplified.